靜態(tài)內(nèi)部類——基于類初始化的延遲加載解決方案
- 創(chuàng)建StaticInnerClassSingleton類
/**
* @author lijiayin
*/
public class StaticInnerClassSingleton {
private StaticInnerClassSingleton(){
}
private static class InnerClass {
private static StaticInnerClassSingleton staticInnerClassSingleton = new StaticInnerClassSingleton();
}
public static StaticInnerClassSingleton getInstance(){
return InnerClass.staticInnerClassSingleton;
}
}
- 創(chuàng)建測試類
/**
* @author lijiayin
*/
public class Test {
public static void main(String[] args) {
Thread t1 = new Thread(() -> {
StaticInnerClassSingleton staticInnerClassSingleton = StaticInnerClassSingleton.getInstance();
System.out.println(Thread.currentThread().getName() + ":" + staticInnerClassSingleton);
});
Thread t2 = new Thread(() -> {
StaticInnerClassSingleton staticInnerClassSingleton = StaticInnerClassSingleton.getInstance();
System.out.println(Thread.currentThread().getName() + ":" + staticInnerClassSingleton);
});
t1.start();
t2.start();
System.out.println("program end");
}
}
-
測試結果
測試結果.png -
類初始化
類初始化.png
餓漢式
- 第一種方式
/**
* @author lijiayin
*/
public class HungrySingleton {
private final static HungrySingleton hungrySingleton = new HungrySingleton();
private HungrySingleton(){
}
public static HungrySingleton getInstance(){
return hungrySingleton;
}
}
- 第二種方式
/**
* @author lijiayin
*/
public class HungrySingleton {
private final static HungrySingleton hungrySingleton;
static {
hungrySingleton = new HungrySingleton();
}
private HungrySingleton(){
}
public static HungrySingleton getInstance(){
return hungrySingleton;
}
}
序列化破壞單例模式原理及解決方案
做一個測試
- 修改HungrySingleton類
/**
* @author lijiayin
*/
public class HungrySingleton implements Serializable {
private final static HungrySingleton hungrySingleton;
static {
hungrySingleton = new HungrySingleton();
}
private HungrySingleton(){
}
public static HungrySingleton getInstance(){
return hungrySingleton;
}
}
- 創(chuàng)建測試類
import java.io.*;
/**
* @author lijiayin
*/
public class SerializableTest {
public static void main(String[] args) throws IOException, ClassNotFoundException {
HungrySingleton hungrySingleton = HungrySingleton.getInstance();
ObjectOutputStream outputStream = new ObjectOutputStream(new FileOutputStream("singleton_file"));
outputStream.writeObject(hungrySingleton);
File file = new File("singleton_file");
ObjectInputStream inputStream = new ObjectInputStream(new FileInputStream(file));
HungrySingleton newInstance = (HungrySingleton)inputStream.readObject();
System.out.println(hungrySingleton);
System.out.println(newInstance);
System.out.println(hungrySingleton == newInstance);
}
}
-
測試結果
測試結果.png - 結論與原因
不是相同的對象
4.1 查看ObjectInputStream的源碼，readObject()方法。
public final Object readObject()
throws IOException, ClassNotFoundException
{
if (enableOverride) {
return readObjectOverride();
}
// if nested read, passHandle contains handle of enclosing object
int outerHandle = passHandle;
try {
Object obj = readObject0(false);
handles.markDependency(outerHandle, passHandle);
ClassNotFoundException ex = handles.lookupException(passHandle);
if (ex != null) {
throw ex;
}
if (depth == 0) {
vlist.doCallbacks();
}
return obj;
} finally {
passHandle = outerHandle;
if (closed && depth == 0) {
clear();
}
}
}
4.2 調(diào)用了readObject0()方法
private Object readObject0(boolean unshared) throws IOException {
boolean oldMode = bin.getBlockDataMode();
if (oldMode) {
int remain = bin.currentBlockRemaining();
if (remain > 0) {
throw new OptionalDataException(remain);
} else if (defaultDataEnd) {
/*
* Fix for 4360508: stream is currently at the end of a field
* value block written via default serialization; since there
* is no terminating TC_ENDBLOCKDATA tag, simulate
* end-of-custom-data behavior explicitly.
*/
throw new OptionalDataException(true);
}
bin.setBlockDataMode(false);
}
byte tc;
while ((tc = bin.peekByte()) == TC_RESET) {
bin.readByte();
handleReset();
}
depth++;
totalObjectRefs++;
try {
switch (tc) {
case TC_NULL:
return readNull();
case TC_REFERENCE:
return readHandle(unshared);
case TC_CLASS:
return readClass(unshared);
case TC_CLASSDESC:
case TC_PROXYCLASSDESC:
return readClassDesc(unshared);
case TC_STRING:
case TC_LONGSTRING:
return checkResolve(readString(unshared));
case TC_ARRAY:
return checkResolve(readArray(unshared));
case TC_ENUM:
return checkResolve(readEnum(unshared));
case TC_OBJECT:
return checkResolve(readOrdinaryObject(unshared));
case TC_EXCEPTION:
IOException ex = readFatalException();
throw new WriteAbortedException("writing aborted", ex);
case TC_BLOCKDATA:
case TC_BLOCKDATALONG:
if (oldMode) {
bin.setBlockDataMode(true);
bin.peek(); // force header read
throw new OptionalDataException(
bin.currentBlockRemaining());
} else {
throw new StreamCorruptedException(
"unexpected block data");
}
case TC_ENDBLOCKDATA:
if (oldMode) {
throw new OptionalDataException(true);
} else {
throw new StreamCorruptedException(
"unexpected end of block data");
}
default:
throw new StreamCorruptedException(
String.format("invalid type code: %02X", tc));
}
} finally {
depth--;
bin.setBlockDataMode(oldMode);
}
}
4.3 當類型為對象時，調(diào)用readOrdinaryObject()方法
private Object readOrdinaryObject(boolean unshared)
throws IOException
{
if (bin.readByte() != TC_OBJECT) {
throw new InternalError();
}
ObjectStreamClass desc = readClassDesc(false);
desc.checkDeserialize();
Class<?> cl = desc.forClass();
if (cl == String.class || cl == Class.class
|| cl == ObjectStreamClass.class) {
throw new InvalidClassException("invalid class descriptor");
}
Object obj;
try {
obj = desc.isInstantiable() ? desc.newInstance() : null;
} catch (Exception ex) {
throw (IOException) new InvalidClassException(
desc.forClass().getName(),
"unable to create instance").initCause(ex);
}
passHandle = handles.assign(unshared ? unsharedMarker : obj);
ClassNotFoundException resolveEx = desc.getResolveException();
if (resolveEx != null) {
handles.markException(passHandle, resolveEx);
}
if (desc.isExternalizable()) {
readExternalData((Externalizable) obj, desc);
} else {
readSerialData(obj, desc);
}
handles.finish(passHandle);
if (obj != null &&
handles.lookupException(passHandle) == null &&
desc.hasReadResolveMethod())
{
Object rep = desc.invokeReadResolve(obj);
if (unshared && rep.getClass().isArray()) {
rep = cloneArray(rep);
}
if (rep != obj) {
// Filter the replacement object
if (rep != null) {
if (rep.getClass().isArray()) {
filterCheck(rep.getClass(), Array.getLength(rep));
} else {
filterCheck(rep.getClass(), -1);
}
}
handles.setObject(passHandle, obj = rep);
}
}
return obj;
}
關注這個地方，當類實現(xiàn)了serializable/externalizable時，就會使用反射創(chuàng)建新的對象obj。
obj = desc.isInstantiable() ? desc.newInstance() : null;
4.4 再看一下這里
if (obj != null &&
handles.lookupException(passHandle) == null &&
desc.hasReadResolveMethod())
當對象obj不為空時，查找是否有readResolve()方法；如果有，則通過反射調(diào)用這個方法創(chuàng)建對象rep；將反射得到的對象rep賦值給對象obj。
如何解決
- 修改HungrySingleton類
import java.io.Serializable;
/**
* @author lijiayin
*/
public class HungrySingleton implements Serializable {
private final static HungrySingleton hungrySingleton;
static {
hungrySingleton = new HungrySingleton();
}
private HungrySingleton(){
}
public static HungrySingleton getInstance(){
return hungrySingleton;
}
private Object readResolve(){
return hungrySingleton;
}
}
-
測試結果
測試結果.png - 結論
是相同的對象
